It has been hypothesized that fatigue and prolactin (PRL) changes during endurance
exercise are influenced by serotonin synthesis, and in turn, release. Such a change
is thought to occur through an increase in blood free tryptophan (TRP) and a concomitant
decrease in those large neutral amino acids (LNAA) which compete with free TRP for
entry into the brain. For further investigation, 10 healthy athletes were randomly
subjected to three test units (TU), each consisting of a treadmill run for 90 min.
The speed was adjusted to a blood lactate level of 2 mmol/l. During the first 30 min
of exercise infusions of 500 ml saline (TU I), 500 ml saline with amino acids (TU
II) or 500 ml saline with 30 U heparin/kg following an oral soy oil solution given
1 h before (TU III) were administered. Rate of perceived exertion (RPE), heart rate
and running speed were recorded during exercise. Venous blood samples were taken after
a 10 h fast, at rest, after 10, 50 and 90 min of exercise as well as 10 and 30 min
post-exercise. PRL, insulin, glucose, ammonia, lactate, triglycerides (TC), free fatty
acids (FFA) and amino acids were determined in each sample. No significant differences
were found im RPE. PRL increased (p < 0.01) in all TU. TC and heparin administration
resulted in an increase (p < 0.01) in FFA, which correlated (p < 0.01) with free TRP
and the ratio of free TRP/TRP. Artificial iincrease in free TRP in TU III did not
affect plasma PRL level. The amino acid infusion in TU II induced an increase in LNAA
but had no significant effect on PRL. PRL and ammonia peaked at the emd of exercise.
We conclude that neither exercise-induced PRL secretion nor RPE are affected by changes
in circulating free TRP and LNAA under the present conditions.
Key words
Free tryptophan - large neutral amino acids - blood-brain barrier - prolactin - fatigue
